Parallel Transmission in MRI using a transmission coil array comprising a set of coils (for example 8, 16, 32 coils) represents an emerging technology, enabling a variety of interesting applications such as RF shimming and Transmit SENSE to improve the performance of MR especially at high field strengths (Katscher U et al. Magn Reson Med. 2003; 49(1):144-5; Zhu Y. Magn Reson Med. 2004; 51(4):775-84). In this context, the transmit coil configuration is of crucial importance for the targeted parallel transmit application, and coil setups with different channel count or coil topology have been proposed (Vernickel P et al. Magn Reson Med. 2007; 58:381-9; Alagappan V et al. Magn Reson Med. 2007; 57:1148-1158; Adriany G et al. Magn Reson Med. 2008; 59:590-597).
However, the optimum coil concept represents a difficult trade-off between various important constraints as e.g. RF power efficiency, specific absorption rate (SAR) properties, time and performance constraints for B1 mapping and shimming with respect to the target application and anatomy.
Throughout this description, B1 mapping is understood as the method of determining transmission coil sensitivities of the transmission coils. Further, shimming is understood as the procedure of adjusting the transmission properties of these coils considering the acquired B1 maps in order to obtain a desired, for example homogeneous transmission profile within a certain spatial MR excitation area in the examination volume.
One example for the above mentioned trade-off between the various constraints is given by the requirement of good transmission coil shimming while ensuring that the SAR level exposed to an object to be imaged is kept at a small level: high quality coil shimming requires the acquisition of B1 maps for each individual coil, which in case of high numbers of transmission coils this requires a substantial amount of time and results in an undesired high SAR level exposed to the object to be imaged.
In practice, clinical parallel transmit applications based on many transmit channels (e.g. N=8) face a variety of problems, such as e.g. cumbersome workflow, difficult SAR control and limiting RF power constraints. For instance, RF shimming based on many transmit channels may result in shim settings with high RF power demands on some of the transmit channels, and hence, increased SAR values. This is due to the fact that some of the coil eigenmodes have little impact on the shimming result. Consequently, the shimmed RF pulses can only be played with low B1, limiting the clinical use for many applications. Regularization techniques can suppress these coil modes, however, this represents typically a careful tradeoff between shimming result and RF power/SAR, which cannot always be done in an automatic fashion. Moreover, the performance of the different coil modes may differ with respect to the target application and anatomy.
In order to provide a solution to this conflict, Nehrke K. and Börnert, P., “Eigenmode Analysis of Transmit Coil Array for Tailored B1 mapping”, MRM 63:754-764 (2010) suggests to use virtual transmit coil arrays, since the linearity of the transmit chain allows the transmit sensitivity matrix to be measured with respect to any virtual coil array originating from superpositions of the actual coil elements via an appropriate transformation matrix. Thus, the linear properties of the MR transmit chain enable the concept of virtual transmit coils. In case the number of virtual coils is chosen to be smaller than the number of physical transmit coils, this virtual transmission coil concept permits performing an accelerated B1 mapping scan since B1 maps from a smaller number of virtual coil elements have to be acquired.